Semiconductor apparatuses conventionally include light-emitting elements, such as light-emitting diodes and semiconductor lasers.
FIG. 5(a) is a diagram conceptionally showing a structure of a light-emitting diode, as an example of a conventional semiconductor apparatus.
As shown in FIG. 5(a), a conventional semiconductor apparatus, such as a light-emitting diode (LED) 220, fundamentally has a laminated structure in which an n-type semiconductor layer 222, an active layer 223, and a p-type semiconductor layer 224 are laminated in order on a substrate 221. A p-type electrode and an n-type electrode (not shown) are formed on a p-type semiconductor layer and an n-type semiconductor layer respectively.
To a light-emitting diode of such a structure, a device structure is applied for taking light generated in a light emitting region with an active layer, from a surface on which an electrode is formed (front surface of the laminated structure), or from a substrate surface which will not grow a semiconductor layer (back or side surface of the laminated structure), in the laminated structure, the generation of light being performed by recombination of a hole introduced from the p-type semiconductor layer 224 to the active layer 223, and an electron from the n-type semiconductor layer 222 to the active layer 223.
In the light-emitting diode, by controlling of the laminated structure at an atomic level, the flatness of the substrate is processed to the level of a mirror surface. Thus, the semiconductor layer, light emitting region and electrode on the substrate are arranged in parallel with one another. Furthermore, the refractive index of the semiconductor layer is greater than the refractive index of the substrate or the electrode (transparent electrode). Thus, a waveguide is formed between the front surface of the p-type semiconductor layer 224 and the front surface of the substrate 221. That is, the waveguide is formed by the structure of the semiconductor layer with a greater refractive index interposed between the substrate and the transparent electrode, which have a smaller refractive index. This waveguide is inserted between the interface of the p-type semiconductor layer and the electrode, and the interface of the substrate and the electrode.
Thus, when light L generated from the active layer enters the surface of the electrode or the surface of the substrate at an angle equal to or greater than a predetermined critical angle, the light L will be reflected from the interface of the electrode and the p-type semiconductor layer 224, or from the front surface of the substrate 221. The light L will then propagate laterally in the laminated structure of the semiconductor layer and will be trapped in the waveguide. Furthermore, the light L will also be lost during the propagation in the lateral direction. As a result, a desired external quantum efficiency (i.e., efficiency to retrieve the light generated within the light-emitting diode, to the outside) cannot be attained. In other words, the light which enters the interface to the substrate or electrode at an angle greater than the critical angle will propagate through the waveguide by repeating total reflection, and the light will be absorbed during the reflection. Because of this, part of the generated light will be attenuated and such light cannot be effectively taken to the outside, resulting in reduced external quantum efficiency.
To cope with such a problem, a method is proposed for forming a concave-convex section on a front surface of a substrate to scatter light generated in a light emitting region, thus improving an external quantum efficiency (see Patent Document 1).
FIG. 6 is a diagram for describing a semiconductor light-emitting element (GaN system LED) disclosed in Patent Document 1. FIG. 6 shows a cross-sectional view of the semiconductor light-emitting element having a substrate with an uneven front surface.
A light-emitting element 210 comprises, as an insulating substrate, a sapphire substrate 211, surface of which is made in a concave-convex shape by forming a plurality of convex sections 211a on the surface. The light-emitting element 210 has a laminated structure obtained by laminating an n-type GaN layer 212, an active layer 213, and a p-type GaN layer 214, on the substrate 211. In the laminated structure, an n-type electrode 217 is formed on an exposed surface of the n-type GaN layer 212, and a p-type electrode (transparent electrode) 216 is formed on the p-type GaN layer 214 with a p-type contact layer 215 interposed therebetween. Furthermore, the entire surface of the light-emitting element is covered by a protection film 218, except for connection sections of electrodes of respective conductivity types, the connection section being connected with wirings.
Next, a manufacturing method will be described.
First, a resist film is patterned on a front surface of the sapphire substrate 211, using a photomask (exposure mask), to form an etching mask. The surface of the sapphire substrate 211 is selectively etched by RIE (reactive ion etching) using an etching mask to form a concave-convex section 211b. At this step, through the patterning of a resist film using the photomask, the exposure process of a resist film is repeatedly performed while moving a wafer stage with a sapphire substrate 211 placed thereon at a constant pitch, thereby forming a repeated pattern on the resist film for forming a concave-convex section on a surface of the sapphire substrate.
Thereafter, an AIN layer (not shown) is formed as a buffer layer, on the sapphire substrate 211 using a sputtering apparatus. An n-type GaN layer 212, an active layer 213, and a p-type GaN layer 214 are successively allowed to grow on the AIN layer, using a MOCVD apparatus.
Furthermore, a p-type electrode 216 is formed, as a transparent electrode, on the p-type GaN layer 214 with a contact layer 215 interposed therebetween. After selective etching of the semiconductor layer on the n-type GaN layer 212 in such a manner to allow part of the surface of the n-type GaN layer 212 to be exposed, an n-type electrode 217 is formed on an exposed surface of the n-type GaN layer 212.
With the light-emitting element 210 having such a structure, its external quantum efficiency is dramatically improved as shown in FIG. 5(b), compared to the light-emitting element 220 (FIG. 5 (a)) with a conventional flat substrate 221.
That is, in the light-emitting element 210 disclosed in FIG. 6, the light L, which laterally propagates in the light-emitting element 220 (FIG. 5(a)) with the conventional flat substrate 221, will be scattered or diffracted at the concave-convex section 211b, and the light generated in the laminated structure of the semiconductor element will be effectively taken from the front surface of the upper semiconductor layer in the laminated structure, or from the back surface of the substrate in the lower part of the laminated structure. As a result, external quantum efficiency can be dramatically improved.
Specifically, first, light flux increases towards the upper side or the lower side with respect to the surface of the substrate by the scattering and diffracting effect of light by the concave-convex section on the surface of the substrate, thereby increasing the luminance of the light emitting surface of the light-emitting element when the light emitting surface is observed from the front (=front luminance). Second, using the scattering and diffracting effect of light by the concave-convex section on the surface of the substrate, the light laterally propagating through the semiconductor layer is decreased, thereby reducing the loss of the light due to absorption during the propagation and increasing the total amount of the light emission.